Abstract: A gas turbine engine for an aircraft includes a combustor with a combustion chamber and fuel spray nozzles to inject fuel into the combustion chamber. The fuel spray nozzles include a first and a second subset of fuel spray nozzles. The combustor is operable in a condition in which each of first subset is supplied with fuel at a greater fuel flow rate than each of the second subset. A ratio of nozzles in the first subset to nozzles in the second subset is 1:3 to 1:6. A first idle-MTO nvPM emissions index ratio is EI idle EI max ? TO . EIidle is the nvPM emissions index of the gas turbine engine operating at around 7% available thrust. EImaxTO is the nvPM emissions index of the gas turbine engine operating at around 100% available thrust. The first idle-MTO nvPM emissions index ratio is less than 3.

Abstract: The application discloses a method of determining an overall fuel allocation for missions performed by at least one aircraft, supplied with fuel from a fuel source comprising an amount of a first fuel and an amount of a different second fuel. The method comprises: determining one or more utility values for each mission of the plurality of missions, wherein the or each utility value for each mission is based on the same one or more of a predicted contrail outcome and a predicted local air quality outcome; and determining the overall fuel allocation for the plurality of missions based on the or each utility value for each mission. Also disclosed is: a method of loading fuel onto one or more aircraft according to the overall fuel allocation; and a fuel allocation determination system.

Abstract: A method of determining an overall fuel allocation and a method of loading fuel onto one or more aircraft according to the overall fuel allocation; and a fuel allocation determination system for missions performed by at least one aircraft, supplied with fuel from a fuel source including an amount of a first fuel and of a different second fuel. The method includes: determining one or more utility values for each mission of the plurality of missions, wherein the or each utility value for each mission is based on the same one or more of a predicted contrail outcome and a predicted local air quality outcome; determining an ordering of the missions of the plurality of missions based on the or each utility value of each mission; and determining the overall fuel allocation for the plurality of missions based on the ordering of the missions.

Abstract: A method of determining an overall fuel allocation for a plurality of missions carried out by at least one aircraft, the missions being supplied with fuel from a fuel source comprising amounts of a first fuel and a different second fuel, is disclosed. The method comprises obtaining an initial proposed fuel allocation for each mission; performing an overall optimisation in which the proposed fuel allocation of at least one mission is modified within the constraints of the total available first and second fuels from the fuel source to maximise an overall utility value over all missions, each mission being associated with a respective mission utility value for a given fuel allocation, the mission utility value being determined according to at least one of a predicted contrail outcome and a predicted local air quality outcome; and determining the overall fuel allocation for the plurality of missions based on the overall optimisation.

Abstract: A power system for an aircraft includes at least one gas turbine engine arranged to burn a fuel so as to provide power to the aircraft; at least one first fuel tank arranged to be used to power ground-based operation of the aircraft; at least one secondary fuel tank arranged to contain a fuel to be used to power the aircraft in flight; and a fuel manager arranged to control fuel supply so as to take fuel from only the at least one first fuel tank to power at least the majority of ground-based operations.

Abstract: A gas turbine engine for an aircraft, comprising: a combustor; a fuel-oil heat exchanger arranged to receive fuel and transfer heat from the oil to the fuel so as to raise the fuel temperature to at least 120° C. on entry to the combustor; a fuel recirculation line arranged to recirculate at least some fuel on a fuel flow path from a first point on the fuel flow path to a second point on the fuel flow path, the second point upstream of the first point; and a modulator valve arranged to modulate the flow of fuel along the fuel recirculation line. A method of operating a gas turbine engine is also disclosed.

Abstract: A gas turbine engine includes a combustor with a combustion chamber and fuel spray nozzles to inject fuel into the combustion chamber. The nozzles include a first and second subset. Each of the nozzles of the first subset is supplied with fuel at a greater flow rate than each of the second subset. A ratio of nozzles in the first subset to the second subset is 1:2 to 1:5. An MTO nvPM emissions index ratio-modified fuel flow is EI maxTO , SAF EI maxTO , FF × W f , maxTO . EImaxTO,SAF is nvPM emissions index in mg/kg of the engine operating at around 100% available thrust with sustainable aviation fuel. EImaxTO,FF is nvPM emissions index in mg/kg of the engine operating at around 100% available thrust with fossil-based hydrocarbon. Wf,maxTO is mass flow rate of fuel to the nozzles in kg/s operating at around 100% available thrust. The MTO nvPM emissions index ratio-modified fuel flow in kg/s is less than 2.

Abstract: A gas turbine engine for an aircraft. The gas turbine engine includes a rich burn, quick quench, lean burn combustor having 14-22 fuel spray nozzles or 2-6 fuel spray nozzles per unit engine core size. A thrust nvPM emissions index ratio is EI maxTO / EI maxTO EI idle / F idle . EIidle is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine operating at around 7% available thrust; EImaxTO is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine operating at around 100% available thrust; FmaxTO is the thrust of the gas turbine engine at around 100% available thrust in kN, Fidle is the thrust of the gas turbine engine at around 7% available thrust in kN; the thrust nvPM emissions index ratio is greater than 0.09; and the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel to the fuel spray nozzles.

Abstract: A gas turbine engine for aircraft includes a combustor with a combustion chamber and fuel spray nozzles to inject fuel into the combustion chamber. The fuel spray nozzles include a first subset and a second subset of nozzles. Each of the first subset is supplied with fuel at a greater rate than each of the second subset. A ratio of the first subset to the second subset is 1:2 to 1:5. A MTO nvPM emissions index ratio is EI maxTO , SAF EI maxTO , FF . EImaxTO,SAF is nvPM emissions index in mg/kg of the engine when operating at around 100% available thrust if fuel provided to the fuel spray nozzles includes sustainable aviation fuel. EImaxTO,FF is nvPM emissions index in mg/kg of the engine when operating at around 100% available thrust if fuel provided to the fuel spray nozzles is fossil-based hydrocarbon fuel. The MTO nvPM emissions index ratio is less than 1.

Abstract: A method of determining one or more fuel characteristics of an aviation fuel suitable for powering a gas turbine engine of an aircraft. The method includes: exposing the surface of a piezoelectric crystal to the fuel; measuring a vibration parameter of the piezoelectric crystal; and determining one or more fuel characteristics of the fuel based on the vibration parameter. Also disclosed is a fuel characteristic determination system, a method of operating an aircraft, and an aircraft.

Abstract: A method of operating a gas turbine engine. The gas turbine engine includes an accessory gearbox arranged to power one or more engine accessories, and a combustor in which fuel is combusted and a fuel-oil heat exchanger arranged to transfer heat between oil and fuel that is provided to the combustor. An average temperature of lubricating oil is at least 180° C. on exit from the accessory gearbox at cruise conditions. The method includes transferring heat from the oil to the fuel before the fuel enters the combustor so as to raise the fuel temperature to an average of at least 135° C. on entry to the combustor at cruise conditions. Also provided is a gas turbine engine.

Abstract: A method of operating a gas turbine engine is disclosed, the gas turbine engine comprising a combustor arranged to combust a fuel; and a fuel management system arranged to provide the fuel to the combustor. The fuel management system comprises two fuel-oil heat exchangers through which oil and the fuel flow, the heat exchangers arranged to transfer heat to the fuel and comprising a primary fuel-oil heat exchanger and a secondary fuel-oil heat exchanger; and a fuel pump arranged to deliver the fuel to the combustor, wherein the fuel pump is located between the two heat exchangers. The method comprises controlling the fuel management system so as to transfer between 200 and 600 kJ/m3 of heat to the fuel from the oil in the primary fuel-oil heat exchanger at cruise conditions.

Abstract: A gas turbine engine includes a staged combustion system having pilot fuel injectors and main fuel injectors. A fuel delivery regulator controls delivery of fuel to the pilot and main fuel injectors, receives fuel from a first fuel source containing a first fuel having a first fuel characteristic and a second fuel source containing a second fuel having a second fuel characteristic. In a transition range of operation between the pilot-only and the pilot-and-main ranges of operation, fuel is delivered to both the pilot and main fuel injectors at a transition staging ratio different from the pilot-and-main staging ratio. The fuel delivery regulator delivers fuel to one or both the pilot and main fuel injectors during the transition range of operation having a different fuel characteristic from fuel delivered to one or both the pilot and main fuel injectors during at least part of the pilot-and-main range of operation.

Abstract: A gas turbine engine for aircraft includes a combustor with a combustion chamber and fuel spray nozzles to inject fuel into the combustion chamber. The fuel spray nozzles include a first subset and a second subset of nozzles. Each of the first subset is supplied with fuel at a greater rate than each of the second subset. A ratio of the first subset to the second subset is 1:2 to 1:5. A MTO nvPM emissions index ratio is EI maxTO , SAF EI maxTO , FF . EImaxTO,SAF is nvPM emissions index in mg/kg of the engine when operating at around 100% available thrust if fuel provided to the fuel spray nozzles includes sustainable aviation fuel. EImaxTO,FF is nvPM emissions index in mg/kg of the engine when operating at around 100% available thrust if fuel provided to the fuel spray nozzles is fossil-based hydrocarbon fuel. The MTO nvPM emissions index ratio is less than 1.

Abstract: A gas turbine engine for an aircraft is disclosed. The gas turbine engine comprises: a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6.

Abstract: A gas turbine engine for an aircraft, comprising: a combustor; a fuel-oil heat exchanger arranged to receive fuel and transfer heat from the oil to the fuel so as to raise the fuel temperature to at least 120° C. on entry to the combustor; a fuel recirculation line arranged to recirculate at least some fuel on a fuel flow path from a first point on the fuel flow path to a second point on the fuel flow path, the second point upstream of the first point; and a modulator valve arranged to modulate the flow of fuel along the fuel recirculation line. A method of operating a gas turbine engine is also disclosed.

Abstract: A gas turbine engine has a first subset of fuel spray nozzles and a second subset of fuel spray nozzles. A combustor is operable with the first subset of fuel spray nozzles supplied with fuel at a greater flow rate than each of the second subset of fuel spray nozzles. A ratio of the first subset of fuel spray nozzles to the second subset of fuel spray nozzles is 1:2 to 1:5. A first idle-MTO nvPM emissions index ratio is: EI idle EI max ? TO , where: EIidle is the nvPM emissions index in mg/kg operating at around 7% available thrust; EImaxTO is the nvPM emissions index in mg/kg of the gas turbine engine operating at around 100% available thrust; and the first idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 60. A sustainable aviation fuel (SAF) can be provided to the fuel spray nozzles.

Abstract: A method of operating a gas turbine engine having a lean burn staged combustion system having a combustor in which fuel is combusted. The gas turbine engine includes a fuel-oil heat exchanger arranged to transfer heat between oil and fuel that is provided to the combustor. The method includes transferring heat from the oil to the fuel before the fuel enters the combustor so as to lower the fuel viscosity to 0.58 mm2/s or lower on entry to the combustor at cruise conditions.

Abstract: A gas turbine engine for an aircraft. The gas turbine engine comprising: a rich burn, quick quench, lean burn (RQL) combustor having a number of fuel spray nozzles in the range of 14-22 or a number of fuel spray nozzles per unit engine core size in the range 2 to 6. An MTO nvPM emissions index ratio is defined as: EI maxTO , SAF EI maxTO , FF where: EImaxTO,SAF is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the fuel spray nozzles comprises a sustainable aviation fuel (SAF); and EImaxTO,FF is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the fuel spray nozzles is a fossil-based hydrocarbon fuel. The MTO nvPM emissions index ratio of the gas turbine engine is less than 1.

Abstract: An engine system comprises a first fuel store, a second fuel store, an engine arranged to produce mechanical power by combustion or oxidation of a fuel in an engine, a fuel distribution system arranged to deliver fuel from the first and second fuel stores to the engine, the first fuel delivered at a first mass flow rate, the second fuel delivered at a second mass flow rate, the first and second mass flow rates contributing to a total mass flow rate of fuel to the engine; and a control system arranged to control the relative fractions of the total mass flow rate of fuel to the engine represented by the first mass flow rate and the second mass flow rate, based on an engine temperature.